Glioblastoma multiforme (GBM), the most aggressive and prevalent manifestation of malignant glioma, are characterized by resistance to extant therapeutic modalities, and exhibit a neurologically debilitating course culminating in death often within 14 months after diagnosis. With this dismal prognosis and a near 100% failure rate of GBM drug development, the critical challenges facing the glioma field are to identify and characterize new drug targets to overcome the notorious therapy resistance of GBM, and to develop drug delivery platforms to target undruggable genetic lesions. Restoration of p53 activity represents an attractive therapeutic strategy for the treatment of GBM, as ~65% of primary GBM patients express functionally defective wildtype p53. Amplification and overexpression of the atypical Bcl2 family protein Bcl2L12 (Bcl2-Like-12) compromises p53 function by blocking the transcriptional activity of p53. To inhibit Bcl2L12 function, we propose to use novel RNAi-based nanoconjugates, termed Spherical Nucleic Acids (SNAs) to neutralize Bcl2L12 expression in established glioma. We have found that Bcl2L12-targeting SNAs (siBcl2L12-SNAs) are able to traverse cellular membranes including the blood-brain-barrier. We established that siBcl2L12-SNAs do not require the use of toxic auxiliary reagents and accumulate effectively in cells and upon crossing of the blood-brain/blood-tumor barrier in intracerebral gliomas upon systemic delivery. They exhibit stability in physiological environments, provoke robust intratumoral Bcl2L12 mRNA and protein knockdown and p53 reactivation, and reduce tumor burden in GBM PDX models. To establish the SNA platform as a p53 activating therapeutic modality applicable to the treatment of other highly malignant and lethal solid cancers, we elected cutaneous melanoma as a second cancer type for the evaluation of siBcl2L12-SNAs. Similar to GBM, cutaneous melanoma are characterized by infrequent p53 mutation and elevated Bcl2L12 expression, which correlates with the degree of melanoma drug resistance and progression. Here, we will test the hypothesis that Bcl2L12 ablation by a high activity SNA conjugate increases p53 tumor suppression, reduces GBM and melanoma progression, and thus represent a novel, broadly applicable therapeutic strategy for the activation of wild-type p53 in solid cancers. In Aim 1, we will determine the mechanism and identify surrogate markers of p53 reactivation by siBcl2L12-SNAs. In Aim 2, we will optimize SNA surface chemistry for optimized delivery of siBcl2L12 oligonucleotides to GBM and melanoma tumors. Aim 3 will evaluate siBcl2L12-SNAs in genetically engineered melanoma (Aim 3a) and GBM mouse models (Aim 3b), as monotherapies, and in combination with the DNA alkylator temozolomide (Aim 3c). The results of this proposal will provide an in-depth characterization of the Bcl2L12 oncoprotein at cellular and biological levels, and will pave the way to successfully implement multi- modal p53 reactivation as therapy into clinical practice.